When the effluent rises it has some velocity, and hence momentum. When it enters the hood it has a tendency to "reflect" and curl downward and potentially escape. If the hood is deep enough (top to bottom) and the hood fan flow is high enough (cfm) the curling effluent will remain captured and will be exhausted. The barrel design in the picture looks to me (without examining one) like it has adequate depth. W.r.t. ya think's question a few messages up: First I would say that the change from vertical to horizontal should be as large a radius curve as you can fit to minimize pressure loss (as well as noise). The effluent momentum (which is a vector) is being changed in direction. This is akin to stopping it and starting it up again. If it can slide along a curve, the momentum loss is significantly less. There is one commercial ducting source I came across once that uses 90-degree transitions, but embeds a curved shape within the duct to improve the flow. I find it hard to answer the question of what questions to ask. Usually, the answers aren't available. Ideally, one would want to know the fan curve (flow vs. static pressure) and try to estimate all the pressure losses (including the pressure one might pull the house down to with or without make-up air) to see what the real maximum flow rate will be. This can then be compared against estimates of effluent generation rate calculated from the reference below. The power used might be adjusted to represent your cooking style, as rarely would one have all burners on maximum and cooking at the highest pan temperature. One thing is certain, flow rate in the hood has to at least equal the flow rate from all the active burners and pans, and arguably should be higher due to imperfect effluent collection. Implied above is that the hood manufacturer can supply pressure loss of the filters vs. flow rate, as well as that of the hood transitions. Getting that data, even if it were ever measured, would be unlikely, me thinks. I suggest that various hoods be "audited" in showrooms to gain a feel for their noisiness versus their designs. The reference worth reading, even if one doesn't intend to use it for calculations, is "Thermal plumes of kitchen appliances: part 2 cooking mode," by Risto, Hannu, and Pekka. (I've found it on-line whenever I've looked for it. Part 1 is also interesting.) If you do use it for calculations, please note the following apparent errors in their Equation 1: The constant should be 0.05 not 5. The units of phi should be kW, not W. (these corrections make their tables consistent with the equation) You will need a scientific calculator or counterpart software to do the exponents (unless you are very old school and know how to do logarithms). You will also need to convert BTU to kW and cubic meters per second to cubic feet per minute. This is left as an exercise for the student. [insert sociopathic laugh here] kas

This is just a guess: I think if you meet the specs and requirements of the remote blower, you should be okay. The important thing is that the size of your duct is at least the size specified for the blower. You might want to confirm that with the blower's manufacturer, if they have a customer service 800#. If your duct size is too small (usually expressed in inches diameter), you will not get the full benefit from the blower, in other words, less than the 1000 cfm. BTW, are you sure you really need all that exhaust capacity? It's really a lot, the sort of thing suitable for a lot of heavy sauteing and grilling, cooking for a big crowd on a regular basis. With all that exhaust capacity, you need to plan also for fresh air intake to compensate for the negative air pressure you will be creating. Plan on having to open a window nearby or, if you really want to get fancy, build in an air intake device inside the venting system. I don't know exactly how they do this -- it's the kind of thing usually installed in restaurants. It's easy to go overboard when planning a renovation. You have to sit back and say, what do I really need, and what will please me?

All of the above may be true, but are not in themselves totally limiting. In general, there is always a blower that will move a desired air flow volumetric rate through a given duct, but one might not want to operate it, listen to it, or pay for it. But in this case we are on the margin, and feasibility is not prohibited using conventional devices. For example, I have a Wolf/Broan 1500 CFM rated roof-mounted blower operating with a 10-inch duct, and given various estimated pressure losses, expect that it moves 900 - 1000 CFM. A 1000 CFM rated typical hood blower, no matter how large the duct, at a minimum will be significantly restricted by the hood baffles, and at a maximum by any added MUA pressure losses, and would be unlikely to move more than 700 CFM in use with a hood. An 8-inch duct has a sectional area of 64% of that of a 10-inch duct, so the pressure losses operating at 67% through an 8-inch duct of the air flow rate through a 10-inch duct will be similar. In general, one wants to select a blower having a fan curve that supports the desired flow rate at the pressure loss that one estimates is present. Without my looking up the specified hood, let us assume that its entrance aperture is 42 x 27 inches or just under 8 sq. ft. This calls for 8 x 90 CFM/sq. ft. or 720 actual CFM. A typical 1000 CFM blower may achieve this with a 8-inch duct, and one certainly can check the assumptions against blower fan curves (where available) to either ensure adequacy, or at least force the MUA system to be active and not cause significant pressure loss. Note that this doesn't violate your appliance guy's opinion about duct flow rates, although I am sure that his view is based on typical in-hood blowers. If you have the room in some part of the duct path, then including a silencer will help reduce noise back at the hood for only a small added pressure loss. In any case, numerous commercial kitchen ventilation blowers can be found that will pull 720 CFM against any pressure loss likely from a well designed hood/MUA setup. It is quieter to move a given flow rate when the fan blades are large and moving slowly than small and moving quickly, as the latter generates more high frequency noise from turbulence. Induction motors can be controlled by rheostat adjusted phase circuits, and these can be put into hoods. My Wolf hood has such a control, and now the technology allows for it to be performed with better techniques at low cost. These comments are meant to touch on most of your questions, but feel free to delve deeper as needed. kas

I also considered kaseki's advice about wiring in a fan control switch and mounting it in its own little box nearby (say on the side of the kitchen cabinet next to the hood) but decided against it. We really don't need a lot of variable speeds for the exhaust, and would rather settle for just a simple On/Off switch that could be mounted on the hood control panel instead of opting for a big fan control mounted in its own receptacle box mounted externally nearby - very much a visible kludgy affair. So I searched around and found, in of all places at Home Depot!, a simple well-known brand On/Off control lever switch - it is Gardner Bender GSW-18, an 8 amp, 125 volt SPST ("Single Pole, Single Throw" = simple "On/Off" switch) with 6-inch wire leads, and fitted for a 1/2" hole. All for less than $4! One note to the wise - the existing hole in the Thermador control panel must be something like 7/16" because the Gardner Bender switch would not fit into the hole as-is. I didn't have a 1/2" drill bit nor a step drill bit to enlarge the existing hole, but then I remembered I had an old tap-and-die set. I pulled out one of the 1/2" diameter taps and after a few minutes of forcefully screwing that tap through the original hole, I was able to actually screw the Gardner switch into the hole. Although it isn't black, and isn't the same type of switch as the existing hood light switches, the new lever switch does what it is supposed to and has made the hood exhaust functional again. There is only one speed now - either on or off. It was relatively simple to wire in -- you just put it in place of the original POTentiometer in the wiring schematic, and the Solid State Fan CONTROL module is just left unconnected under the hood - not used anymore with the simple Gardner On/Off switch. Bottom line - thanks kaseki for all of the advice and imput, couldnt have done it without your expertise and help! Plus, got a 22-23 year-old Thermador hood exhaust fan running again for $4 -- a lot less than the $130 replacement fan switch (of which none are available), or the replacement cost for a new hood! This is what it looks like now, rather purposeful and industrially high-tech in its own way ;-D: